Up-to-date fingerprint matching systems using fingerprint image transfer into electronic data usually apply the known contact method to create a fingerprint pattern. A surface topography of a finger is approximated by a series of ridges with intermediate valleys. When a finger is applied to a surface of a transparent optical plate or prism, the ridges contact the optical plate while the valleys do not and instead serve to form the boundaries of regions of air and/or moisture.
The finger to be imaged is illuminated by a light source located below or near to the optical plate. Imaging light from the light source is incident on the surface of the optical plate at an angle of incidence measured with respect to a normal to that surface. Imaging light reflected from the surface is detected by an imaging system that usually includes some form of a detector.
Components of a typical fingerprint imaging system are oriented so that an angle of observation (defined to be an angle between an optical axis of the imaging system and the normal to the optical plate surface) is greater than a critical angle for the interface between the surface and the air at the surface. The critical angle at the surface/air interface is defined as the smallest angle of incidence for which imaging light striking the surface/air interface is totally internally reflected (TIR) within the optical plate. Therefore, the critical angle at the surface/air interface depends on the index of refraction of the air and the optical plate. Another constraint for the angle of observation arises because there is incentive to observe the image at the smallest practical angle of observation, as this reduces distortion due to object tilting. Therefore, the angle of observation is typically chosen to be close to, but greater than the critical angle at the surface/air interface.
At locations where the ridges of the finger contact the surface of the optical plate, total internal reflection does not occur because the index of refraction of a finger is larger than that of air. In this case, light incident on the surface of the optical plate at a location where the ridge of the finger contacts the surface is refracted through the surface/finger interface and then scattered in transmission and reflection upon contact with the finger. In this case, only a small fraction of incident illumination is reflected back to a detector of the imaging system.
The imaging system may be implemented to produce bright components at valley locations and dark components at ridge locations, thus producing a dark or positive fingerprint pattern. Here, the imaging system detects the light reflected from the surface/air interface and is called specular or bright field illumination. Alternatively, the imaging system may be implemented to produce bright components at ridge locations and dark components at valley locations, thus producing a bright or negative fingerprint pattern. In this case, the imaging system detects a small percentage of the illumination that is diffused upon contact with the finger, which is a dark field illumination scheme.